Ancient Carbon Haze Offers Clues to Galaxy Evolution

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Astronomers have detected carbon smog permeating the interstellar
atmospheres of early galaxies, helping confirm that these ancient
galaxies were mostly dust free.

The discovery sheds light on how some of the first galaxies to
form in
the universe grew and evolved, researchers said.

Gas and dust are the main components of the interstellar medium,
the matter in galaxies that constitutes the building blocks of
stars and planets. The gases hydrogen and helium make up 98
percent of all "normal" (i.e., not dark) matter in the universe.
The other 2 percent — any elements heavier than helium, making up
everything from dust to planets — was created from the fusion of
hydrogen and helium atoms in the hearts of stars. [ The
History & Structure of the Universe (Infographic) ]

Since dust formed only after the birth of the first stars,
scientists expected that galaxies began
nearly dustless, getting dustier as they evolved. However,
confirming what the interstellar medium in early galaxies was
like has been a challenge for researchers. If there was less
dust, it would make starlight from early galaxies bluer, but
there are many other effects that could have made this light
bluer as well, said study lead author Peter Capak, an astronomer
at the California Institute of Technology in Pasadena.

"There is little dust in early galaxies about 1 billion years
old, in the epoch of the first galaxies," Capak told Space.com.
"We are starting to see the point where galaxies are truly
primordial, some of the first to form stars."

The researchers analyzed nine star-forming galaxies located about
13 billion light-years away from Earth. The scientists therefore
viewed the galaxies when the universe was 1 billion years old or
so, or only about seven percent of its current age.

The team focused on the faint glow of ionized carbon. This
element can become ionized, or electrically charged, due to the
powerful ultraviolet radiation emitted by bright, massive stars.
In the process, it will give off specific frequencies of radio
waves.

Since carbon has a strong chemical affinity for other elements,
binding to make simple and complex organic molecules, it does not
remain in an unbound, ionized state for very long. This means the
radio glow of ionized carbon is probably a good marker for an
early galaxy, which would possess much lower concentrations of
the heavy elements that ionized carbon would bind to in later
galaxies to form dust grains.

Prior attempts to detect the radio glow of ionized carbon in
early galaxies had failed for nearly 30 years. Some researchers
had speculated that a few billion years more were needed for
stars to manufacture sufficient quantities of carbon to be seen
across vast cosmic distances.

However, ALMA readily detected the haze of ionized carbon in
these early galaxies. In comparison, similar galaxies existing
about 2 billion years later had much less ionized carbon. Prior
studies failed to detect this ancient radio glow, researchers
said, because these studies focused on atypical
galaxies undergoing mergers — dramatic activity that may have
masked the faint signal from ionized carbon.

By analyzing how ionized carbon is moving in these early
galaxies, astronomers can learn details about star formation at
the time, Capak added. This, in turn, could help solve the
mystery of how galaxies were able to reach the massive sizes they
attained early in the universe's history, he said.

The scientists detailed their findings online today (June 24) in
the
journal Nature.